Bilateral capacity for speech sound processing in auditory comprehension: evidence from Wada procedures

Area of Science:

• Neuroscience
• Cognitive Psychology
• Speech Perception

Background:

• Lesion studies indicate temporal lobe systems in both hemispheres support speech sound perception.
• Patients with left temporal lobe damage (Wernicke's aphasics) show surprising word comprehension, with semantic errors predominating.
• The role of the right hemisphere in speech processing—whether due to plasticity or inherent ability—remains unclear.

Purpose of the Study:

• To investigate whether the right hemisphere's speech sound processing ability stems from neural plasticity or inherent function.
• To differentiate between semantic and phonemic errors in auditory comprehension following acute hemispheric deactivation.

Main Methods:

• Wada procedures were used to temporarily deactivate either the left or right hemisphere in 20 patients.
• Participants performed an auditory comprehension task, identifying a target word from visual options (target, semantic foil, phonemic foil, unrelated foil).
• Auditory comprehension was assessed under baseline, left carotid injection, and right carotid injection conditions.

Main Results:

• Left hemisphere deactivation resulted in a significantly higher error rate compared to right hemisphere deactivation.
• The majority of errors (75%) during left hemisphere deactivation were semantic, not phonemic.
• Auditory comprehension deficits were predominantly semantic, even with acute left hemisphere disruption.

Conclusions:

• The findings support the hypothesis that the right hemisphere possesses inherent speech sound processing capabilities.
• Acute left hemisphere disruption primarily impacts semantic aspects of auditory comprehension.
• The right hemisphere's contribution to speech perception is present in the intact brain, not solely a result of compensatory plasticity.